1. Technical Field
The invention is related to high frequency capacitively coupled radio frequency (RF) plasma reactors for semiconductor wafers and in particular to improvements in the control of the RF source power level and to protection of the wafer pedestal from damage during chamber cleaning operations.
2. Background Art
Referring to FIG. 1A, a plasma chamber 10 operating at a high RF frequency (e.g., 13.56 MHz) holds a semiconductor wafer or substrate (not shown) and performs a desired process on the wafer, such as plasma etching, plasma-enhanced chemical vapor deposition, or sputtering. In order to maintain a plasma within the chamber 10, RF power is furnished to the chamber 10 by a conventional 13.56 MHz RF generator 12 via a transmission line 14 (such as a 50 ohm coaxial cable) and a variable RF match circuit 16. The conventional RF generator 12 includes a 13.56 MHz RF power source 12a, an output stage 12b, and a conventional dual directional coupler 12c. The output stage 12b matches the non-standard output impedance of the RF power circuit 12a to the industry standard characteristic impedance of the coaxial cable of 50 ohms. The dual directional coupler 12c provides outputs 18a, 18b indicative of the forward and reflected R.F power at the output of the output stage 12b. A conventional feedback control circuit 20 governs the variable RF match circuit in such a manner as to minimize the VSWR, that is, to minimize the reflected RF power sensed at the reflected power output 18b of the dual directional coupler. Specifically, the control circuit adjusts the variable RF match network so as to transform the impedance of the plasma chamber to the 50 ohm characteristic impedance of the coaxial cable. Since the plasma chamber impedance tends to vary during wafer processing, the variable RF match circuit 16 maintains an RF match by changing its impedance to compensate for fluctuations in the plasma chamber impedance. The variable RF match circuit and its feedback control circuit to maintain a low VSWR are essential for three reasons: (1) It provides a constant load impedance to the output stage of the RF generator, so that the generator will supply a constant RF power level to the plasma chamber. Fluctuations in the RF power level would cause instability and unpredictability in the semiconductor processing performed by the plasma chamber. (2) It avoids the high peak voltages associated with a high VSWR on the transmission line 14, which high voltages could damage the transmission line. (3) It avoids the high peak currents associated with a high VSWR on the transmission line, which currents could cause overheating of the transmission line which could change its characteristic impedance, thereby causing nonuniform chamber process conditions depending on the temperature history of the transmission line.
One problem with the system of FIG. 1 is that the variable RF match circuit 16 is expensive and bulky. Furthermore, variable RF match networks which employ moving parts tend to be unreliable. However, at the high RF power levels of 1200 Watts required for chemical vapor deposition processes or 2000 Watts required for etch or chamber cleaning operations, the variable RF match network 14 has seemed indispensable at high RF frequencies on the order of 13.56 MHz.
Referring to FIG. 1B, a low frequency (e.g., 350 kHz) RF generator 12' has a low frequency power circuit 12a' in lieu of the high frequency power circuit 12a of FIG. 1A. The low frequency RF generator 12' is coupled through a fixed RF match network 30 to the plasma chamber 10. Because the RF frequency is so low, the transmission line between the RF generator and the plasma chamber generally is much shorter than a quarter wavelength, so that no standing wave pattern occurs in the transmission line. Therefore the above-mentioned problems (2) and (3) of excessive peak voltages and peak currents will not occur even if there is a mismatch between the plasma chamber impedance and the transmission line.
However, item (1), the effect of plasma chamber load impedance changes on RF power source output level, still must be addressed. In one known low frequency (350 kHz) RF generator design shown in FIG. 1B, a control circuit 32 dynamically adjusts the power level of the RF power circuit 12a to maintain the delivered RF power (i.e., the difference between the forward power 18a and reflected power 18b) at a constant level. Since this control circuit makes the RF power level independent of moderate changes in load impedance, this design omits the variable RF match circuit and its feedback control circuit 20, and substitutes a fixed RF match network 30. The fixed RF match network 30 has a fixed output impedance which is selected to match the average impedance of the plasma inside the chamber 10 over time. Since the plasma impedance will necessarily fluctuate, there will be RF mismatching and loss of RF power due to power reflection back to the RF generator 12. This contrasts with the approach of FIG. 1A in which the power level of the RF circuit 12a is maintained at a constant level and fluctuations in the plasma impedance are compensated by changing the impedance of the variable match circuit 16 to minimize the reflected RF power.
Since the transmission line 14 is short compared to a quarter wavelength, it does not matter which end of the transmission line the match network is connected to, so for convenience the fixed RF match circuit 30 can be included as a component within the RF power generator 12'.
The control circuit 32 includes a first subtractor node 34 which senses delivered RF power (i.e., the difference between the forward power and reflected power outputs 18a, 18b of the dual directional coupler 12c) and a second subtractor node 36 which senses the difference between the delivered RF power and a desired RF power set by the user. (In the diagram of FIG. 2, the desired power level is depicted as a variable analog power source 38 applied to one input of the subtractor node 36, although the source 38 may be digital and the comparison may be carried out digitally by the subtraction node 36.) The output of the second subtractor node is applied as an error signal to a control input of the RF power circuit 12a. The RF power circuit 12a responds to the error signal by either increasing or decreasing the output power level, depending upon the sign of the error signal. This servos the power output level of the RF power circuit 12a to minimize or null the error signal so that the delivered power is maintained at the desired power level. The error signal applied to the control input of the RF power circuit 12a is zero in the absence of fluctuations in the plasma impedance.
The low frequency RF plasma system of FIG. 1B advantageously avoids the need for an expensive, potentially unreliable, variable RF match circuit. However, it has not seemed possible to avoid the use of a variable RF match circuit at higher RF frequencies, such as the commonly used frequency of 13.56 MHz.